With the ongoing deployment of long term evolution (LTE) and LTE-Advanced cellular systems (hereafter LTE for conciseness), an increasing number of smartphones are reaching the market every year. As smartphone penetration has resulted in drastic increase in cellular network traffic loads, service providers (SPs) are facing the challenge of scarce spectrum resources (mainly between 700 MHz and 2.6 GHz). As a possible solution to this problem, some manufacturers and service provider (SPs) have considered extending the new-generation of LTE systems to unlicensed bands (2.4 and 5 GHz) to leverage additional spectrum resources, leading to so-called unlicensed LTE (U-LTE). U-LTE will operate pico/femto cells in unlicensed bands with transmission power levels much lower than typically used in macro cells.
However, extending LTE to unlicensed bands is by no means trivial, primarily because of the lack of compatibility between the radio resource management (RRM) schemes used by LTE and by systems already deployed in unlicensed bands (e.g., Wi-Fi). The underlying RRM policy of LTE systems is in fact based on centralized scheduling on each LTE base station (referred to as eNodeB). This is different from typical systems with distributed control that are already deployed in unlicensed bands, e.g., IEEE 802.11-based (Wi-Fi) systems with RRM based on distributed coordination function (DCF) running in all wireless stations (STAs). It has been observed that the throughput of Wi-Fi systems can be considerably degraded (more than 50% under high traffic loads) in the presence of co-channel interference from LTE systems, which is not desirable especially for SPs who have deployed tens of thousands of Wi-Fi hotspots over the world. To date, there is still no widely accepted coexistence scheme to enable spectrally-efficient and fair spectrum sharing between LTE and Wi-Fi.